(1) Field of the Invention
This invention relates to an electroluminescent device with excellent luminescence properties.
(2) Description of the Prior Art
An organic electroluminescent device (hereinafter, referred to as an xe2x80x9cEL devicexe2x80x9d) is a self-light emitting device utilizing the principle that applying an electric field to a fluorescent substance causes its light emission through recombination energy of positive holes injected from an anode and electrons from a cathode. Since, based on a laminated device, C. W. Tang et al of Eastman Kodak Company reported an organic EL device driven by a low voltage (C. W. Tang, S. A. VanSlyke, Applied Physics Letters, Vol. 51, 913 (1987)), an organic EL device composed of organic materials has been intensively investigated. Tang et al. have used tris(8-hydroxyquinolinol) aluminum in a light-emitting layer and a triphenyl diamine derivative in a hole-transporting layer. A layered structure has advantages such as an improved efficiency of injecting positive holes into a light-emitting layer; an improved efficiency of generating excitons obtained from recombination, by blocking electrons injected from a cathode; and confinement of excitons generated in a light-emitting layer. As is shown above, well-known structures for an organic EL device include a two-layer type comprising a hole-transporting(injection) layer and an electron-transporting light-emitting layer, and a three-layer type comprising a hole-transporting(injection) layer, a light-emitting layer, and an electron-transporting(injection) layer. In these layered structures of devices, various device structures and manufacturing processes have been devised for improving an efficiency of recombination of injected positive holes and electrons.
As hole-transporting materials for these devices, triphenylamine derivatives and aromatic diamine derivatives such as 4,4xe2x80x2,4xe2x80x3-tris(3-methylphenylphenylamino)triphenylamine, which is a star-burst molecule, and N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-bis(3-methylphenyl)-[1,1xe2x80x2-biphenyl]-4,4xe2x80x2-diamine are well-known (e.g., Japanese Patent Laid-Open Nos. 20771/1996, 40995/1996, 40997/1996, 53397/1996, 87122/1996).
As electron-transporting materials, oxadiazole derivatives and triazole derivatives are well-known.
Known electron-transporting light-emitting materials are chelate complexes such as tris(8-quinolinolate) aluminum complex, coumarin derivatives, tetraphenylbutadiene derivatives, bisstyryl arylenes and oxadiazole derivatives. It has been reported that they may give a light-emitting color in the visible region from blue to red, and thus they are promising for realizing a color-display device (e.g., Japanese Patent Laid-Open Nos. 239655/1996, 138561/1995 and 200289/1991).
There have been recently disclosed or reported organic EL devices with a high luminance and a long life time, which are, however, not necessarily satisfactory. It has been, therefore, desired to develop materials having a high performance.
In view of the above problems, an objective of this invention is to provide an organic EL device with an improved luminance.
We have intensely investigated for solving the above problems, and have obtained the following observations on an organic EL device prepared by using particular tetrabenzopentacene derivatives having a diarylamino substituent. It has been found that the material has a good carrier transport property. It has been also found that an organic EL device prepared by using the material as a hole- or electron-transporting material, or by using a mixture layer of the above material and another hole- or electron-transporting material, may have a higher luminance than a conventional one. Furthermore, it has been found that an organic EL device may give a particularly high luminance, when employing the above diarylamino-substituted tetrabenzopentacene derivatives in which the aryl group is substituted with a styryl group as a light-emitting, hole-transporting or electron-transporting material.
This invention may be specified by the following items (1) to (8).
(1) An organic electroluminescent device having one or more organic thin layers including a light-emitting layer between an anode and a cathode, wherein at least one organic thin layer contains a material represented by the general formula (1) in a form of a mixture or a single substance: 
wherein R1 to R18 independently represent hydrogen atom, halogen atom, hydroxy group, a substituted or unsubstituted amino group, nitro group, cyano group, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted cycloalkyl group, a substituted or unsubstituted alkoxy group, a substituted or unsubstituted aromatic hydrocarbon group, a substituted or unsubstituted aromatic heterocyclic group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted aryloxy group, a substituted or unsubstituted alkoxycarbonyl group, or carboxyl group. Any two of R1 to R18 may be combined together to form a ring.
At least one of R1 to R18 is a diarylamino group represented by xe2x80x94Nar1Ar2 wherein Ar1 and Ar2 independently represent a substituted or unsubstituted aryl group having 6 to 20 carbon atoms.
(2) An organic electroluminescent device having one or more organic thin layers including a light-emitting layer between an anode and a cathode, wherein at least one organic thin layer contains a material represented by the general formula (2) in a form of a mixture or a single substance: 
wherein R1 to R18 independently represent hydrogen atom, halogen atom, hydroxy group, a substituted or unsubstituted amino group, nitro group, cyano group, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted cycloalkyl group, a substituted or unsubstituted alkoxy group, a substituted or unsubstituted aromatic hydrocarbon group, a substituted or unsubstituted aromatic heterocyclic group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted aryloxy group, a substituted or unsubstituted alkoxycarbonyl group, or carboxyl group. Any two of R1 to R18 may be combined together to form a ring. provided that at least one of R1 to R18 is a diarylamino group represented by xe2x80x94NAr1Ar2 wherein Ar1 and Ar2 independently represent a substituted or unsubstituted aryl group having 6 to 20 carbon atoms and at least one of Ar1 and Ar2 is substituted with a substituted or unsubstituted styryl group.
(3) The organic electroluminescent device described in (1) wherein the light-emitting layer contains the compound represented by the general formula (1) in a form of a mixture or a single substance.
(4) The organic electroluminescent device described in (2) wherein the light-emitting layer contains the compound represented by the general formula (2) in a form of a mixture or a single substance.
(5) The organic electroluminescent device described in (1) wherein the organic film layer contains at least a hole-transporting layer comprising the compound represented by the general formula (1) in a form of a mixture or a single substance.
(6) The organic electroluminescent device described in (2) wherein the organic film layer contains at least a hole-transporting layer comprising the compound represented by the general formula (2) in a form of a mixture or a single substance.
(7) The organic electroluminescent device described in (1) wherein the organic film layer contains at least an electron-transporting layer comprising the compound represented by the general formula (1) in a form of a mixture or a single substance.
(8) The organic electroluminescent device described in (2) wherein the organic film layer contains at least an electron-transporting layer comprising the compound represented by the general formula (2) in a form of a mixture or a single substance.
An organic electroluminescent device according to this invention, at least one of its organic film layers comprises a particular compound represented by the general formula (1) or (2), has higher luminance than that of a conventional organic electroluminescent device.